Positive working imaging systems in which an originally coloured species is decolourised in an image-wise manner are known. These systems have the advantage of giving a positive copy of an original. One of the earliest forms of positive working imaging systems was developed utilising the properties of photographic silver, e.g. as disclosed in British Patent Specification No. 17773 (1889), Austrian Patent Specification No. OE42478 and B. Gaspar, Zeitschrift Wiss. Phot. 34, 119 (1935). Since then many forms of colour silver halide photography have been developed.
Silverless dye bleaching processes are also known, but in spite of the apparent simplicity of these systems, they have encountered a number of problems. The inadequate photosensitivity of such systems consisting of colour layers, the lack of purity and stability of the white in the final print and difficulty of finding dyes which form a neutral grey and bleaching at equal rates, are some of the problems. Early systems are disclosed in Smith, Photogr. J., April 1910, page 141. More recently, cyanines with borate anions are disclosed as a dye bleach system in British Patent Specification Nos. 1 370 058, 1 370 059 and 1 370 060. A dye bleach process involving tetra(alkyl)borate is disclosed in U.S. Pat. No. 4,307,182 and fixing methods are disclosed in European Patent Specification No. 0040978. U.S. Pat. No. 3,595,655 discloses a silverless dye bleach system consisting essentially of a polymethine dye and an activator which is a carbonyl, azo, diazo, organic-sulphur containing or peroxide compound.
It is an object of the present invention to provide new radiation-sensitive elements capable of recording a positive image.